Information about the co-added images supplied by Gemini

For MOS programs scheduled during the current semester, the PIs
will receive separate instructions by e-mail on how to retrieve the
reduced and co-added images of their targets from the Gemini Science Archive. Due to the short time
available to design and prepare the masks, it is strongly recommended
to use these images as the basis for the mask designs, rather than to
start from the raw imaging data.

The reduced and co-added images have been produced using the
Gemini IRAF package which includes reduction software for GMOS.
The software correctly takes into account the gaps between the CCDs and the
misalignment of the CCDs relative to each other.

The images have been co-added with the task imcoadd, which is
also part of the Gemini IRAF package. The co-added images are
average images, thus they are scaled to the exposure time of the individual images
used to create the co-added image (see the keyword EXPTIME in the header of the
primary header unit). Further, the co-added images are in e-/pixel.

How to prepare an Object Table

The Object Table is a FITS table that contains the following mandatory columns:

ID

Unique object id (integer)

RA

RA in hours (real)

DEC

Dec in degrees (real)

x_ccd

X coordinate of object position in pixels (real)

y_ccd

Y coordinate of object position in pixels (real)

MAG

(Relative) magnitude of object (real)

priority

Priority of object (char*1; "0/1/2/3/X")

The order of the columns is important. The column names are case sensitive.

The priority column is a single character with 5 possible values. Priority 0 is reserved for alignment stars, and priority X designates objects that are ignored by the mask making software. More information on how the mask making software uses priorities is available here

Other columns may be present in the table and may help the user selecting objects
for the mask. The user may find the following columns useful:

slitsize_x

Slit width in arcsec (real)

slitsize_y

Slit length in arcsec (real)

slittilt

Slit position angle in degrees (real)

slitpos_y

Slit position in the Y-direction (spatial) relative to the object position in arcsec (real)

The default is slittilt=0.
slittilt is measured counter-clockwise relative to the default
position.
The column slitpos_y is useful for designing slits where the objects
are not all in the centers of the slits. Tweaking the slit positions in the
Y-direction may allow the user to fit more slits in a given mask.
The example script objtexample2.cl shows
the use of the columns slitsize_x, slitsize_y,
slittilt, and slitpos_y in more detail.

NOTE: If the image is binned, the Object Table MUST contain a header
keyword "PIXSCALE" containing the pixel scale in arcsec/pixel.

Two IRAF tasks app2objt and stsdas2objt are supplied to help
turn the output from common photometry programs into valid Object Tables.

app2objt - Convert a table from daofind/apphot/daophot into an Object Table
stsdas2objt - Convert FITS or STSDAS tables into an Object Table

These tasks are part of the released version of the Gemini IRAF package
and can be found in the package gemini.gmos.mostools (ie. the user needs to load the packages "gemini", "gmos" and "mostools").

The task app2objt may be used to convert the output from the IRAF
tasks daofind, phot, allstar or nstar
into valid Object Tables.

The task stsdas2objt may be used to convert any FITS or STSDAS
table into a valid Object Table. The task is specifically designed to
handle FITS output tables from SExtractor.

The user may also prepare the Object Table with any other software capable of
writing FITS tables. In this case, it is recommended to process the FITS table
with the task stsdas2objt to ensure that the column names and the format
is correct.

To make the preparation of the Object Table go a little easier, the user may
find the following two example IRAF scripts useful:

objtexample2.cl - How to prepare an Object Table from output from SExtractor

Making the best use of imaging from other telescopes

If the user has prior imaging from other telescopes, it may be possible to use
GMOS images for the mask design obtained with quite short exposure times and/or in
worse seeing conditions than needed for the spectroscopy.
If the GMOS images contain a sufficient number of objects (> 50) with good signal-to-noise
then these objects may be used to "boot-strap" the pixel positions of the science targets.

It is recommended that the user proceeds as follows.

Based on the bright (but unsaturated) objects in the GMOS image, determine the
transformation between GMOS pixel coordinates (x_ccd,y_ccd) and the (RA,DEC) from
the previous observations.

Transform all (RA,DEC) from the previous observations to GMOS pixel coordinates
(x_ccd,y_ccd).

Make an STSDAS table or a FITS table that has the following columns

ID

Unique object id (integer)

RA

RA a in hours (real)

DEC

Dec in degrees (real)

x_ccd

X coordinate of object position in pixels (real)

y_ccd

Y coordinate of object position in pixels (real)

MAG

(Relative) magnitude of object (real)

priority

Priority of object (integer)

plus any other columns the user would like to include

The table may be made from ASCII input using the task tcreate in the IRAF package
tables.ttools which is part of the STSDAS software distributed by STScI.

Process the table with the task stsdas2objt to ensure that a valid
Object Table is produced. It is recommended to recalculate (RA,DEC) using the World
Coordinate System in the image headers. To do so, set the flag fl_wcs to "yes" in
the task stsdas2objt. (RA,DEC) are used for plotting only, while (x_ccd,y_ccd)
is used to define the positions of the slit.